IB DP Biology Topic 2: Molecular biology 2.6 Structure of DNA and RNA Question Bank SL Paper 2

Question

a.DNA research, involving biotechnology, has led to benefits for society but has given rise to some controversy.

b.Outline how translation depends on complementary base pairing.[3]

c. Describe the polymerase chain reaction (PCR), including the role of Taq DNA polymerase.[4]

 

Explain benefits and risks of using genetically modified crops for the environment and also for human health.[8]

▶️Answer/Explanation

Ans:

a.

a. translation converts a sequence of mRNA nucleotides/codons to a sequence of amino acids/polypeptide/protein

b. «triplets of» nucleotides/bases on «activated» tRNAs pair with complementary «triplets of» nucleotides/bases on mRNA / vice versa

c. base pairing occurs when adenine/A pairs with uracil/U and guanine/G pairs with cytosine/C

d. specific amino acids are attached to specific of tRNA

e. mRNA has codons AND tRNA has anticodons

a. PCR is process by which a small sample of DNA can be amplified/copied many times

b. PCR involves repeated cycling through high and lower temperatures «to promote melting and annealing of DNA strands»

c. «mixture» is heated to high temperatures to break «hydrogen» bonds between strands of DNA/to separate the double-stranded DNA

d. Taq DNA polymerase can withstand high temperatures without denaturing

e. primers bind to «targeted» DNA sequences at lower temp

f. Taq DNA polymerase forms new «double-stranded» DNA by adding «complementary» bases/nucleotides

Environment benefits:

a. pest-resistant crops can be made

b. so less spraying of insecticides/pesticides

c. less fuel burned in management of crops

d. longer shelf-life for fruits and vegetables so less spoilage

e. greater quantity/shorter growing time/less land needed

f. increase variety of growing locations / can grow in threatened conditions

Environment risks:

g. non-target organisms can be affected

h. genes transferred to crop plants to make them herbicide resistant could spread to wild plants making super-weeds

i. GMOs (encourage monoculture which) reduces biodiversity

j. GM crops encourage overuse of herbicides

Health benefits:

k. nutritional value of food improved by increasing nutrient content

l. crops could be produced that lack toxins or allergens

m. crops could be produced to contain edible vaccines to provide natural disease resistance

Health risks:

n. proteins from transferred genes could be toxic or cause allergic reactions

o. antibiotic resistance genes used as markers during gene transfer could spread to «pathogenic» bacteria

p. transferred genes could cause unexpected/not anticipated problems
OR
health effects of exposure to GMO unclear

 
 

Question

Native oyster populations are decreasing where rivers meet the ocean along the northwest coast of North America. These oyster populations are being attacked by a gastropod.

It is known that oysters and gastropods have hard parts composed of calcium carbonate and that ocean acidification is increasing. Studies were carried out using juvenile oysters and gastropods to investigate the effects of acidification on the decrease in the population of oysters.

The first step was to raise oysters in two different mesocosms. One had seawater at a normal concentration of $CO_2$ and the other had sea water with a high concentration of $CO_2$. Gastropods were raised in two further mesocosms with normal and high$CO_2$ concentrations respectively.

A juvenile gastropod will attack a juvenile oyster by using its tongue-like structure (radula) to drill a hole through the oyster shell. Once the hole has been drilled, the gastropod sucks out the soft flesh. Researchers investigated the shell thickness at the site of the drill hole in relation to the size of the oyster. The results are seen in this graph.

Equal numbers of oysters raised in seawater with a normal$CO_2$ concentration and in seawater with a high $CO_2$ concentration were then presented together to the gastropod predators in seawater with a normal $CO_2$ concentration. The same numbers of oysters from the two groups were also presented together to the gastropods in seawater with a high $CO_2$ concentration. The bar charts show how many of the oysters were drilled by the gastropods and the mean size of drilled oysters.

Outline how acidified sea water could affect the shells of the oyster.[1]

a.

Outline the trends shown in the data in the graph.[2]

b.

Estimate how much smaller drilled oysters raised in seawater at a high CO2 concentration were than drilled oysters raised in seawater at a normal CO2 concentration.[1]

c.

Deduce from the data in the bar charts which factors were and were not correlated significantly with the number of oysters drilled by the gastropods.[2]

d.i.

Suggest reasons for the differences in the numbers of oysters drilled, as shown in the bar charts.[2]

d.ii.

The radula in a gastropod is hard but not made of calcium carbonate. Outline how this statement is supported by the drilling success of the gastropods in seawater with normal or high $CO_2$ concentrations.[2]

d.iii.

Using all the data, evaluate how $CO_2$ concentrations affect the development of oysters and their predation by gastropods.[2]

Shells might dissolve/deteriorate / become smaller/thinner/weaker / OWTTE
OR
shell formation reduced / more difficult

▶️Answer/Explanation

Markscheme

a.
a. positive correlation between shell thickness and shell size
OR
as shell thickness increases, shell size «also» increases
b. (positive correlation) occurs at two different \$CO_2\$ concentrations / both high and normal concentrations
c. trend for thickness is «slightly» lower with high $\$ \mathrm{CO} 2 \$$
b.
«approximately» $0.2 \mathrm{~mm} 2$
OR
«approximately» $40 \%$ «smaller»
unit required
c.
a. significant factor: concentration of $\$ \mathrm{CO} 2 \$$ in which oysters were raised
b. insignificant factor: concentration of $\$ \mathrm{CO}_{-} 2 \$$ at which oysters were presented to gastropods
d.i.
a. (because) shells are thinner/smaller when the oyster is raised in high $\$ \mathrm{CO} 2 \$ /$ lower pH
OR

«because» lower pH/higher acidity prevents/reduces deposition of calcium carbonate

b. gastropods target smaller/thinner-shelled oysters more

c. gastropods can eat/drill thin-shelled/smaller oysters at a faster rate (and move onto another)

d. eating smaller oysters «from high $CO_2$ environments» means given population of gastropods require more oysters for same food intake

d.ii.

a. the data/trend lines indicate that a higher $CO_2$ concentration diminishes the shell thickness, making gastropod predation more successful
OR
the bar graphs suggest that oysters raised in a higher $CO_2$ concentration are smaller, making gastropod predation more successful

b. $CO_2$ concentrations «during feeding» do not change the occurrence of drilling/predation «by gastropods»

c. «limitation» no information about how exaggerated the $CO_2$ concentrations were
OR
«limitation» no information about numbers of gastropods used «in each setting»

Question

a.Draw a labelled diagram of a section of DNA showing four nucleotides.[5]

b. Outline a technique used for gene transfer.[5]

c.Explain how evolution may happen in response to an environmental change.[8]

▶️Answer/Explanation

Markscheme

a.)Award [1] for each labelled item shown correctly connected.

b.

a. plasmid used for gene transfer/removed from bacteria;

b. plasmid is a small/extra circle of DNA;

c. restriction enzymes/endonucleases cut/cleave DNA (of plasmid);

d. each restriction enzyme cuts at specific base sequence/creates sticky ends;

e. same (restriction) enzyme used to cut DNA with (desired) gene;

f. DNA/gene can be added to the open plasmid/sticky ends join gene and plasmid;

g. (DNA) ligase used to splice/join together/seal nicks;

h. recombinant DNA/plasmids inserted into host cell/bacterium/yeast;

 

a. (genetic) variation in population;

b. (variation is) due to mutation / sexual reproduction;

c. valid example of variation in a specific population;

d. more offspring are produced than can survive / populations over-populate;

e. competition / struggle for resources/survival;

f. example of competition/struggle for resources;

g. survival of fittest/best adapted (to the changed environment)/those with beneficial adaptations / converse;

h. example of changed environment and adaptation to it;

i. favourable genes/alleles passed on / best adapted reproduce (more) / converse;

j. example of reproduction of individuals better adapted to changed environment;

k. alleles for adaptations to the changed environment increase in the population;

l. example of genes/alleles for adaptations increasing in a population;

m. evolution by natural selection;

n. evolution is (cumulative) change in population/species over time / change in allele frequency;

Suitable examples are antibiotic resistance and the peppered moth but any genuine evidence-based example of adaptation to environmental change can be credited.c.

Question

a.Draw a labelled diagram of the structure of DNA, showing the arrangement of subunits.[3]

b. Explain DNA replication.[3]<

▶️Answer/Explanation

Markscheme


a.
correctly shows two antiparallel sugar-phosphate strands/backbones with linkages between phosphates and sugars connected through bases; (phosphate and simple names such as sugar and base are acceptable labels. They must be given at least once.)
correctly labeled phosphate and deoxyribose and base;
sugar linked to phosphates through correct pentagon corners/(5’–3’) linkages;
shows complementary base pairs of A-T/Adenine–Thymine and G-C/Guanine–Cytosine;
correctly indicates both covalent/phosphodiester and hydrogen bonds.
b.)
DNA replication is semi-conservative/daughter DNA molecule contains one parent strand and one new strand;
unwinding of double helix/separation of two strands by helicase;
separated (parent) strands become templates for new strands;
free/single nucleotides join (parent/template) strands through complementary base pairing;
DNA polymerase joins nucleotides in new strands;
Award [3] for the above points clearly shown in an annotated diagram.

Question

The diagram below represents a DNA nucleotide.

Identify the phosphate group and deoxyribose.

a.

Phosphate group: ………………………………………….
Deoxyribose: ………………………………………………..[1]

b. Draw a labelled diagram to show how four nucleotides are joined together to form a double-stranded DNA molecule with two base pairs.[3]

c.State two differences between RNA and DNA nucleotides.[2]

▶️Answer/Explanation

Ans:

a.Phosphate: I
Deoxyribose: III
Both correct for one mark.

a.

b. two sugar-phosphate strands shown connected through bases;
a sugar-phosphate bond labeled as a covalent bond;
hydrogen bonds labeled on line between bases;
boxes labeled as (nitrogenous) bases;
complementary base pairing/A–T/G–C;
(5’–3′) linkages correctly shown; (no label required)

c RNA nucleotides contain ribose and DNA nucleotides contain deoxyribose; (some) RNA nucleotides contain uracil and (some) DNA nucleotides contain thymine;
 

Question

a.Draw a labelled diagram of the molecular structure of DNA including at least four nucleotides.[5]

b.A small DNA sample found at a crime scene can be used in an investigation. Describe the steps taken in the processing of this small sample of DNA.[6]

c.Discuss the relationship between one gene and one polypeptide.[7]

▶️Answer/Explanation

Markscheme

a.)The diagram must show four nucleotides shown with two on each side showing phosphate-sugar backbones and nitrogen base pairs bonded between them.

Award [1] for each of the following clearly drawn and correctly labelled.
phosphate – shown connected to deoxyribose;
deoxyribose – shown connected to phosphate;
(nitrogenous) bases – shown bonded to deoxyribose;
base pairs – shown with labels adenine/A bonded to thymine/T and cytosine/C bonded to guanine/G;
hydrogen bonds – shown connecting bases;
covalent bonds – shown connecting deoxyribose to phosphates;
nucleotide – clearly identified by the candidate;
Award [4 max] if diagram is not shown double stranded.

b.)DNA samples are taken from crime scene, suspects and victims;
polymerase chain reaction/PCR used to increase the amount of DNA;
restriction enzymes used to cut DNA;
electrophoresis involves electric field/placing sample between electrodes;
used to separate DNA fragments according to size;
creating DNA profiles/unique patterns of bands;
comparison is made between the patterns;
criminals/victims can be identified in this way;
DNA is (quite) stable / DNA can be processed long after the crime;

c..)DNA codes for a specific sequence of amino acids/polypeptide;
the DNA code for one polypeptide is a gene;
DNA is transcribed into mRNA;
mRNA moves to a ribosome;
where mRNA is translated into a polypeptide;
originally it was thought that one gene always codes for one polypeptide;
some genes do not code for a polypeptide;
some genes code for transfer RNA/tRNA/ribosomal RNA/rRNA;
some sections of DNA code for regulators that are not polypeptides;
antibody production does not follow this pattern (of simple transcription-translation); (allow other examples)
change in the gene/mutation will affect the primary structure of the polypeptide;

Question

a. Outline the bonding between DNA nucleotides.[2]

b.Explain how chemical bonding between water molecules makes water a valuable coolant in living organisms.[2]

c.Describe the movement of water across membranes.[2]

d.Outline the role of water in photosynthesis.[2]

▶️Answer/Explanation

Markscheme
a.)hydrogen bonds between nucleotides of opposite strands/complementary bases/adenine and thymine and cytosine and guanine;
covalent bonds between nucleotides within strands/between sugar/deoxyribose and phosphate;
b.)hydrogen bonding between water molecules;
breaking (hydrogen bonds) needs/removes energy/heat;
hydrogen bonds must break when water evaporates/vaporizes;
c.)osmosis / moves passively;
from regions of low solute/high water potential/concentration to high solute concentration / low water potential/concentration;
passes through protein channels/aquaporins/selectively-permeable membrane;
d.)water molecules undergo photolysis/are split by light energy;
forms oxygen as a by-product;
hydrogen helps power the fixation of carbon (into organic molecules);

Question

a.Outline the bonding between DNA nucleotides.[2]

b.Explain how chemical bonding between water molecules makes water a valuable coolant in living organisms.[2]

c.Describe the movement of water across membranes.[2]

d. Outline the role of water in photosynthesis.[2]

▶️Answer/Explanation

Markscheme
a.)hydrogen bonds between nucleotides of opposite strands/complementary bases/adenine and thymine and cytosine and guanine;
covalent bonds between nucleotides within strands/between sugar/deoxyribose and phosphate;
b.)hydrogen bonding between water molecules;
breaking (hydrogen bonds) needs/removes energy/heat;
hydrogen bonds must break when water evaporates/vaporizes;

c.)osmosis / moves passively;
from regions of low solute/high water potential/concentration to high solute concentration / low water potential/concentration;
passes through protein channels/aquaporins/selectively-permeable membrane;
d.)water molecules undergo photolysis/are split by light energy;
forms oxygen as a by-product;
hydrogen helps power the fixation of carbon (into organic molecules);

Question

a.State the type of bonds that

(i) connect base pairs in a DNA molecule.

(ii) link DNA nucleotides into a single strand.[2]

b.Distinguish between DNA and RNA nucleotides by giving two differences in the chemical structure of the molecules.[2]

c.Explain the role of transfer RNA (tRNA) in the process of translation.[2]

▶️Answer/Explanation

Markscheme
a.) (i) hydrogen

(ii) covalent / phosphodiester linkage
b.)DNA has deoxyribose, RNA has ribose;

DNA has base T/thymine, RNA has base U/uracil;

Do not accept double or single strands as chemical structure.

c.)c. tRNA attaches to (specific) amino acid;

tRNA (with amino acid) moves to the ribosome;

anticodon of tRNA binds with codon of mRNA;

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